A wind turbine, like some other structures such as aircraft propellers, fans, and the like, includes blades configured for rotating about an axis. Typically, two or more blades are provided each coupled to a rotatable hub. A number of hub configurations can be used and a number of wind turbine designs can be used, including that described in U.S. Pat. application Ser. No. 08/360,310, filed Dec. 21, 1994, entitled Rotor Device and Control for Wind Turbine, incorporated herein by reference. In the course of operation, each blade transfers a load to the hub. This load has a number of components such as an axial component, a twist component, a bending component, and the like. For certain designs, the bending component of the load transferred from the blade to the hub has been particularly problematic. The bending component is the component in a direction substantially normal to the blade longitudinal axis. The difficulties of bending load transfer occur in many types of designs, but are especially acute in configurations where the blade and the hub have different compositions (such as a composite blade and a metallic hub).
One previous approach, depicted in FIG. 1, provides inner and outer steel rings 110, 112, coupled to the root end of the composite material blade 114, e.g., via an adhesive bond and/or precision diameter bolts 116 installed in match drilled radial holds around the perimeter. The structure is coupled to a hub via bolts 118 attached to threaded retention holes 120. It is believed that the bending load transfer mechanism in this design results principally in a shearing effect, at the interface between the concentric steel rings 110, 112 and the main structural spar 114 of the blade. To achieve a degree of reliability, this design involves providing high precision in the dimensions of the spar root end 114, particularly the inner and outer diameters thereof which, in a practical sense, limits the design of the root end to a circular cross section, since such high precision is difficult to achieve in other shapes such as, for example, elliptical sections. It is believed that, for at least some designs, if a high degree of precision is not achieved, the adhesive bond or other coupling of the root end of the blade to the rings may be (locally) faulty and loss of structural integrity could result.
FIG. 2 depicts, in cross section, a similar configuration with a cylindrical section composite blade root 214, bonded adhesively and coupled via pins 216 in an annular region defined by inner and outer steel annular plates 210, 212, with hub connection being achieved via bolts threaded through holes 220, in a flange 222. This design is believed also to rely principally on shear strength of the adhesively bonded joint and to involve a high degree of precision in the dimensions of the blade root 214, particularly the inner and outer diameters. The high precision involved in the designs depicted in FIGS. 1 and 2, is believed to add undesirably to the overall cost of the device.
Another previous configuration is depicted generally in FIG. 3. In this design, the root end of the blade 314 is compressed axially against a hub or hub extension 324, by a plurality of the steel studs or bolts 318, extending through axial holes 326, molded or machined into the blade root end, and engaging steel inserts or cylindrical shaped "barrel nuts" 316 positioned in radial holes formed in the root end 314. To effect a transfer of bending moment from the blade root end 314 to the hub or hub extension 324, this configuration is believed to lead to high localized contact stresses at the interface between the barrel nuts 316 and the surrounding blade root material. Such high localized stress may, it is believed, lead to local crushing of the blade material, potential loosening of the studs or bolts 318, and eventual loss of structural integrity. A modified version of the configuration of FIG. 3 involves embedding the studs 318 directly in the blade material. In this modification, high localized contact stresses are believed to occur between the stud thread surfaces and the composite material, similarly providing a risk of localized crushing of blade material and loss of structural integrity. It is believed that some previous designs, as a result of the potential for high localized stresses, have provided blades which are relatively large in diameter, and which have relatively thick or heavy wall sections throughout the root area, in order to accommodate such high local stresses.
Accordingly, it would be advantageous to provide for a blade design which achieves effective transfer of bending loads from the blade to the hub, while having a relatively lower cost, lower requirement for precision in the shape of the blade root end, greater opportunity for designs other than circular cross-section designs, reduction or elimination of high localized stresses, and ability to provide for blades with relatively small root and diameters and thinner wall sections in the root area.